1. Field of the Invention
This invention relates to an imaging lens installed in an imaging apparatus using an imaging device such as a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), etc.
2. Description of Related Art
In recent years, miniaturization of an imaging device of a CCD, a CMOS, etc., and providing the imaging device with a larger number of pixels have very advanced. Thus, small size and high performance are also demanded for the imaging apparatus main unit and lenses installed therein. To be compatible with an imaging device with a large number of pixels, telecentric property is also demanded so that the incidence angle of the principal ray on an imaging device becomes close to a parallel to the optical axis (the incidence angle on the imaging surface is close to zero with respect to the normal to the imaging surface). Hitherto, for example, an imaging lens installed in a mobile phone with a camera often has been made up of three lenses, but the number of the lenses tends to further increase to be compatible with a larger number of pixels. JP-A-2005-208236 describes an imaging lens made up of four lenses. In the imaging lens described in JP-A-2005-208236, a second lens placed from the object side is a negative lens with a concave surface directed to the object side.
However, there is still scope for improvement for the imaging lens described in JP-A-2005-208236 from the viewpoint of chromatic aberration, etc. To ensure the telecentric property, it is considered that an aperture diaphragm is placed close to the object side as much as possible. In this case, if the surface of the second lens on the object side thereof is concave like the imaging lens described in JP-A-2005-208236, the manufacturing sensitivity becomes high and, for example, if a lens position shift occurs at the manufacturing time, image surface variation easily occurs.